Jazzy, that bites. I'm not sure what to tell you because I don't do anything special to make sure the cookie doesn't get overwritten by someone else. I don't use the remote order form or anything. I just link to them just like you've been. So I'm sure the same thing happens to me too, but due to volume I just don't track everything closely. As you know, that's what happened when I referred Erika, but the only reason I found out was because I knew her personally and I knew to look for her name. Most people that buy SBI! from my links don't contact me first so I'm at the mercy of technology like everyone else. Unfortunately, it would be impossible for me to know how many sales went to other affiliates.
You might want to try only using the links that have a no-overwrite in place. That means if they visit your affiliate link and then accidentally buy from someone else's site you still get the sale (unless they've cleared their cookies or something, but of course you have no control over that.)
You'll have to go to the Promotion Center to find out which links have that (I can't remember) but I know affiliates.sitesell.com is one. But they have to join the affiliate program first. This will ensure they are tied to you forever. I think
http://mycps.sitesell.com is another landing page with no-overwrite status.
And speaking of cookies, I'm wondering if that's what has been happening. More and more computers have those automated "clean up" systems where cookies are cleared on a weekly basis (I know mine is setup to empty my cache once a week, but I leave the cookies alone). So I wonder if that is coming into play here.